Field of the Invention
The invention relates to a leadframe of a conductive material, having a central region to accommodate a chip, and a plurality of connecting fingers that extend at least from one side in the direction of the central region, a contact region being provided adjacent to the central region on at least some of the connecting fingers. The invention relates further to a component including a chip, a leadframe of a conductive material, a central region and connecting fingers, and a housing of a molding compound, the chip having contact pads on its upper side that are connected electrically to the connecting fingers, and the housing enclosing the chip and regions of the connecting fingers.
Components of the above-mentioned type are also called Surface Mounted Device (SMD) components. There is a large number of different housing configurations, which are of different sizes and have a different number of connecting fingers. SMD components permit standardized production and automatic population of printed circuit boards to which they are applied.
A typical structure of an SMD component is described below. First, a chip is connected to a system carrier. The system carrier is frequently constructed as a leadframe and has a central region to accommodate a chip and a plurality of connecting fingers, which extend from at least one side in the direction of the central region. In some leadframes, the central region is configured as an island-like chip carrier. The chip is fitted to this chip carrier, for example, by adhesive bonding, soldering, or alloying, and connected firmly thereto. In another variant, the connecting fingers, themselves, form the central region. In such a case, the connecting fingers extend from at least two opposite sides in the direction of the central region. The chip is fixed to the leadframe such that an adhesive layer, generally in the form of an adhesive strip, is applied to the active main side of the chip. Regions of the connecting fingers are brought into contact with the adhesive to achieve retention of the chip. Such a configuration is also referred to as Lead On Chip (LOC). Its identifying feature is that the connecting fingers partly overlap the chip face, by which mechanical fixing of the chip is achieved. After the chip has been connected to the leadframe, its individual contact pads are connected to contact regions on the connecting fingers. The electrical connection is produced, for example, by bonding wires. The chip and the connecting fingers of the connecting frame are, then, encapsulated such that the chip is surrounded completely by a molding compound and the connecting fingers project to some extent out of the housing. The housing normally is of a curable plastic molding compound, which is processed at about 175 degrees C. The regions of the leadframe that are not needed are, then, removed by punching.
During the production of the component, during the fitting of the latter to a printed circuit board and during operation, both are subjected to high loadings. During production, the component is subjected to mechanical loadings as a result of bending the regions of the connecting fingers that are located outside the housing, or by punching out the parts of the leadframe that are not needed. When the component is soldered onto a printed circuit board, it is subjected to temperatures in the range between 220° and 240° C. Thus, thermal-mechanical stresses act on the component. During the operation of the component itself, high temperatures, likewise, arise that cause thermal stresses. These stresses arise as a result of the fact that the materials of the leadframe, of the housing, and of the chip, itself, have different coefficients of thermal expansion. Because of such a characteristic, shear stresses arise in the interior of the component. These shear stresses can cause delamination at the points at which leadframe material meets housing material. Furthermore, cracks can occur in the housing and in the chip. As a result of penetrating moisture, the component can, then, be impaired in terms of its ability to function. Cracks in the chip can lead to functional failure.
During the design of the components, attempts are, therefore, made to achieve neutral stress states as far as possible during manufacture by appropriate material selection, by the design of the leadframe, and by the process configuration. In particular, the configuration of the leadframe has a decisive influence on the ruggedness of the component with regard to process-induced stresses or forces on the housing, which are viewed as a latent cause for housing fractures that continually occur.
To avoid such difficulties, “downsets,” as they are referred, are often provided. Such a configuration is used almost exclusively in leadframes with a chip island. In this case, the island-like chip carrier is offset upward or downward with respect to the connecting fingers. After the chip has been fitted and the housing has been provided, the chip is surrounded on its upper and lower side by a housing cover of equal thickness. By such measures, it is possible to reduce bending of the housing that is caused by different coefficients of expansion of the materials used (chip, plastic molding compound). The downset is provided regularly only for small chips.
Otherwise, the focus of the configuration of the leadframes lies in their electrical performance. The leadframes, that is to say, in particular, the shape of the connecting fingers, are optimized to short signal propagation paths and to a connection configuration in relation to the chip that is optimum for wire bonding. Leadframes that are provided for use in the aforementioned LOC housings are likewise configured only with regard to their electrical performance.